Make JavaScript HttpConnection emit timeout event (#3)

The JavaScript `HttpConnection` allows the caller to set a request timeout via the `nodeOptions` property. However, neither the HTTP connection nor the thrift client provides a way for the caller to know when a timeout occured. With this change, the `HttpConnection` will now emit a timeout event when the NodeJS timeout event occurs. A caller can add an event listener to the connection as follows:

```
const thrift = require('thrift');
const connection = thrift.createHttpConnection('localhost', 1234, {
  nodeOptions: {
    timeout: 1000
  }
});
connection.on();
connection.on('timeout', () => {
  console.log('Request timed out after 1000ms');
});
```

lib/nodejs/lib/thrift/http_connection.js

@@ -224,6 +224,9 @@ HttpConnection.prototype.write = function(data) {
   req.on('error', function(err) {
     self.emit("error", err);
   });
+  req.on('timeout', function(err) {
+    self.emit("timeout", err);
+  });
   req.write(data);
   req.end();
 };